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In Situ Measurements of Nonlinear and Nonequilibrium Dynamics in Shallow, Unconsolidated Sediments

ExxonMobil Upstream Research Company, Houston, Texas 77252-2189

Paul Bodin

Pacific Northwest Seismic Network, Department of Earth Sciences, University of Washington, Seattle, Washington 98195-7010

Charles A. Langston

Center for Earthquake Research and Information, University of Memphis, Memphis, Tennessee 38152-3050

We present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow, unconsolidated sediments. An array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g. We determined high-strain Rayleigh-wave dispersion across the array in the band from 5 to 60 Hz. Rayleigh-wave phase velocities at frequencies above 10 Hz are wave amplitude-dependent and a function of the driving-force amplitude; as driving force was increased phase velocity decreased, consistent with nonlinear dynamics. We demonstrate the existence of a temporary, nonequilibrium state occurring in the near-surface soils during and after the induced nonlinear behavior. Nonlinear conditioning is demonstrated by measuring changes in Rayleigh-wave phase velocity for input signals with the same applied driving-force amplitude. A logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle. Measurements from weak-motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible. We demonstrate that an active source, field-based approach has the potential to expand our knowledge of how sediments respond to strong ground motions, provide additional insight into the poorly understood slow dynamic recovery process, and possibly even lead to a new, site-specific, and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits.